This invention relates in general to voltage controlled oscillators and relates in particular to a voltage controlled ring oscillator that can operate at high frequencies and that can be implemented in a small number of elements formed on a single chip. Voltage controlled oscillators are widely used in communication systems. In FIG. 1 is illustrated a conventional ring oscillator. This oscillator consists of an odd number N of inverters, each having a gate delay of T.sub.d. If a rising logic edge propagates around the ring it will, after one round trip, become a falling edge. After two such trips, it will again be a rising edge. Since the time of propagation around the loop is N*T.sub.d, the period of this signal will be 2N*T.sub.d. Equivalently, this circuit oscillates at a frequency of 1/2N*T.sub.d.
Three conditions are required to ensure that this circuit will function as an oscillator. First, the small signal gain of each inverter must be such that the net gain around the loop is greater than 1. When the net gain is greater than 1, the signal on the loop will grow with each pass around the loop until the inverters saturate to a point such that the total gain of the loop drops to 1. Second, there must be an odd number of inversions around the loop. If there were an even number, then the circuit operation could peg at the maximum positive or negative signal achievable at saturation of all inverters and the circuit would not oscillate. Thus, by requiring a net inversion around the ring, it is assured that such saturation at a constant circuit value will not occur. An alternate means of preventing latchup is to provide for an AC coupling in the ring. This method is usually avoided in monolithic IC implementations due to the difficulty in fabricating the required coupling capacitors. Third, N is chosen to be greater than two. According to Nyquist's Stability Criterion, the existence of more than two dominant poles in the circuit transfer function guarantees that the circuit will be unstable. Therefore, N is chosen to be greater than two.
In conventional voltage controlled ring oscillators, the frequency is varied by varying the gate delay T.sub.d of each stage. In one such oscillator, an FET is introduced before each inverter. The control voltage is applied to the gate of each of the FETs to vary the delay of each FET, thereby varying the frequency of the oscillator. In another voltage controlled ring oscillator, voltage controlled bias currents are used for each inverter to modify the rise and fall times of the inverters, thereby varying the gate delay T.sub.d of each inverter. Unfortunately, both of these schemes introduce elements that limit the maximum frequency that can be produced by the oscillator.
For cost reasons, it is always desirable to minimize both the chip count and the number of external parts needed to build a given system. Thus, it would be advantageous to have an oscillator design that utilizes a small number of elements that are compatible so that they can be integrated onto a single chip with other circuits with which the oscillator is to interact without need for external frequency determining elements.